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Abstract:

A clayish composition for forming a sintered silver alloy body capable of
forming a sintered silver alloy body, which is not easily discolored even
in the atmosphere and has excellent tensile strength, flexural strength,
surface hardness (hereinafter, sometimes collectively referred to as
`mechanical strength`), elongation or the like, powder for the clayish
composition for forming a sintered silver alloy body, a method for
manufacturing the clayish composition for forming a sintered silver alloy
body, a sintered silver alloy body and a method for manufacturing the
sintered silver alloy body.

Claims:

1-15. (canceled)

16. A composition for forming a sintered silver-copper alloy body
consisting of: a powder constituent consisting of silver powder and
copper oxide powder; a binder; and water, wherein, the powder constituent
includes copper (II) oxide powder (CuO powder) as the copper oxide powder
in a range of from 4 mass % to 35 mass % with respect to the entire
powder constituent, and the amount of elemental silver powder is from 46
mass % to 97 mass % with respect to the entire metal elements in the
powder constituent.

17. The composition for forming a sintered silver-copper alloy body
according to claim 16, wherein the copper oxide powder includes copper
(I) oxide, and the total amount of copper (II) oxide and copper (I) oxide
in the powder constituent is 54 mass % or less with respect to the entire
powder constituent.

18. The composition for forming a sintered silver-copper alloy body
according to claim 16, wherein the average particle diameter of the
copper oxide powder is from 1 μm to 25 μm.

19. The composition for forming a sintered silver-copper alloy body
according to claim 16, wherein the binder includes at least one kind or
two or more kinds of binders selected from the group consisting of a
cellulose-based binder, a polyvinyl compound-based binder, an acrylic
compound-based binder, a wax-based binder, a resin-based binder, starch,
gelatin and flour.

20. Powder for the composition for forming a sintered silver-copper alloy
body, consisting of: silver powder; and copper oxide powder, wherein, the
powder includes copper (II) oxide powder (CuO powder) as the copper oxide
powder in a range of from 4 mass % to 35 mass % with respect to the
entire powder, and the amount of elemental silver powder is from 46 mass
% to 97 mass % with respect to the entire metal elements in the powder.

21. The powder for the composition for forming a sintered silver-copper
alloy body according to claim 20, wherein the powder includes CuO powder
as the copper oxide powder in a range of from 12 mass % to 35 mass % with
respect to the entire powder, and the amount of elemental silver powder
is from 46 mass % to 90 mass % with respect to the entire metal elements
in the powder.

22. The powder for the composition for forming a sintered silver-copper
alloy body according to claim 20, wherein the copper oxide powder
includes copper (I) oxide, and the total amount of copper (II) oxide and
copper (I) oxide in the powder is 54 mass % or less with respect to the
entire powder.

23. The powder for the composition for forming a sintered silver-copper
alloy body according to claim 20, wherein the average particle diameter
of the copper oxide powder is from 1 μm to 25 μm.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a clayish composition for forming
a sintered silver alloy body, a powder for the clayish composition for
forming a sintered silver alloy body, a method for manufacturing the
clayish composition for forming a sintered silver alloy body, a sintered
silver alloy body obtained from the clayish composition for forming a
sintered body, and a method for manufacturing the sintered silver alloy
body.

[0003] In the past, silver-made jewelry, artistically crafted items, and
the like represented by, for example a ring or the like, have been
manufactured by, in general, casting or forging a silver-containing
material. However, in recent years, silver clay (clayish composition for
forming a sintered body) including silver powder has become commercially
available, and a method is suggested that manufactures silver jewelry or
artistically crafted items having an arbitrary shape by making the silver
clay into an arbitrary shape and then firing the silver clay (for
example, refer to Patent Document 1). According to such a method, silver
clay can be freely shaped like general clay is shaped, therefore
silver-made jewelry, artistically crafted items and the like can be
manufactured in an extremely simple manner by drying a shaped body
obtained by shaping and then firing the shaped body using a furnace.

[0004] Meanwhile, the silver clay described in Patent Document 1 is, in
general, obtained by adding a binder or water, and, as a necessity, a
surface active agent or the like to the powder of pure silver (pure Ag)
and then kneading the mixture. However, in a case in which silver clay is
made using silver powder of pure Ag and then heated so as to manufacture
a silver sintered body, there is a problem in that the obtained silver
sintered body has poor strength characteristics since the strength of
pure Ag itself is weak.

[0005] To solve the above-described problem of the strength
characteristics, another method is also suggested that manufactures a
silver sintered body, which is a so-called sterling silver, by shaping
and then firing silver clay obtained by constituting a silver powder with
a silver alloy including Ag in a component ratio of 92.5% and,
furthermore, copper (Cu) or the like, and adding the silver powder to a
binder or the like and kneading the mixture (for example, refer to the
`Example` section or the like in Patent Document 2).

Patent Document

[0006][Patent Document 1] Japanese Patent Publication No. 4265127

[0007][Patent Document 2] Japanese Patent Publication No. 3274960

DISCLOSURE OF INVENTION

Problems to be Solved by the Invention

[0008] However, even if silver clay made of sterling silver, which is an
Ag--Cu alloy, has an improved strength characteristics compared with a
silver sintered body using the silver powder of pure Ag as described in
Patent Document 2, there is a problem in that the hue of the silver clay
is liable to degrade since Cu included in the silver clay may be easily
altered. Specifically, in a case in which the silver clay made of
sterling silver is kept at room temperature in the atmosphere, it is
observed that the silver clay may already be discolored at a point in
time several days after the manufacturing date of the silver clay, and
not only the surface but also the inside is discolored.

[0009] The present invention has been made in consideration of the above
problem, and the object of the present invention is to provide a clayish
composition for forming a sintered silver alloy body capable of forming a
sintered silver alloy body which is not easily discolored even in the
atmosphere and has excellent tensile strength, flexural strength, surface
hardness (hereinafter, sometimes, collectively referred to as `mechanical
strength`), elongation or the like, powder for the clayish composition
for forming a sintered silver alloy body, a method for manufacturing the
clayish composition for forming a sintered silver alloy body, a sintered
silver alloy body and a method for manufacturing the sintered silver
alloy body.

Means for Solving the Problem

[0010] The inventors of the present invention have conducted thorough
studies in order to solve the above problem and found that the
discoloration of silver clay (clayish composition for forming a sintered
silver alloy body) can be suppressed by constituting powder for silver
clay (powder for the clayish composition for forming a sintered silver
alloy body), which constitutes silver clay (clayish composition for
forming a sintered silver alloy body), with powder including silver
powder and copper oxide powder.

[0011] The present invention has been made based on the above founding and
includes the constitution shown below. [0012] (1) The clayish composition
for forming a sintered silver alloy body according to the present
invention is characterized by including a powder constituent including
silver powder and copper oxide powder, a binder and water.

[0013] The clayish composition for forming the sintered silver alloy body
with such a constitution includes the silver powder, the copper oxide
powder, the binder and water. Here, the copper oxide is chemically stable
compared with metallic copper, thereby having a less possibility of being
easily altered (change in the valence of copper ions) in the atmosphere.
Therefore, the discoloration of the clayish composition for forming a
sintered silver alloy body can be suppressed.

[0014] Furthermore, since the binder in the clayish composition for
forming a sintered silver alloy body can be combusted and thus removed by
using oxygen in the copper oxide, it is possible to accelerate sintering.
[0015] (2) Here, the clayish composition for forming the sintered silver
alloy body according to (1) preferably includes at least copper (II)
oxide powder (CuO powder) as the copper oxide powder.

[0016] Since the clayish composition for forming the sintered silver alloy
body with this constitution includes the copper (II) oxide powder, which
is chemically stable, the discoloration of the clayish composition for
forming the sintered silver alloy body can be reliably prevented.

[0017] In addition, the binder in the clayish composition for forming the
sintered silver alloy body can be combusted and thus removed by using the
oxygen in CuO. Therefore, even in a relatively thick object with a
thickness of 5 mm or more, the binder can be combusted inside the object
by using the oxygen of CuO, it is therefore possible to manufacture a
high-quality sintered silver alloy body. [0018] (3) In addition, in the
clayish composition for forming the sintered silver alloy body according
to (1) or (2), the powder constituent preferably includes CuO powder as
the copper oxide powder in a range of from 4 mass % to 35 mass % with
respect to the entire powder constituent, and the amount of Ag element is
preferably from 46 mass % to 97 mass % with respect to the entire metal
elements in the powder constituent.

[0019] If the amount of CuO powder is less than 4 mass %, the mechanical
strength may not be sufficiently improved. On the other hand, if the
amount of CuO powder exceeds 35 mass %, the elongation degrades and a
sintered silver alloy body made by using the powder for silver clay may
not exhibit a beautiful silver color even after polishing. Consequently,
the amount of CuO powder is preferably in a range of from 4 mass % to 35
mass %. [0020] (4) Furthermore, in the clayish composition for forming
the sintered silver alloy body according to any one of (1) to (3), the
powder constituent preferably includes CuO powder as the copper oxide
powder in a range of from 12 mass % to 35 mass % with respect to the
entire powder constituent, and the amount of Ag element is preferably
from 46 mass % to 90 mass % with respect to the entire metal elements in
the powder constituent.

[0021] In the case of the amount of CuO powder of 12 mass % or more, the
binder included in the clayish composition for forming the sintered
silver alloy body can be combusted and thus removed by using the oxygen
of CuO. Therefore, pre-firing is not necessary to remove the binder in
advance, and it is possible to conduct a drying treatment after making
and then conduct firing. [0022] (5) In addition, in the clayish
composition for forming the sintered silver alloy body according to any
one of (1) to (4), the powder constituent further includes metallic
copper, and the amount of the metallic copper in the powder constituent
is preferably 2 mass or less with respect to the entire powder
constituent.

[0023] By containing 2 mass % or less of the metallic copper in the powder
constituent with respect to the entire powder constituent, the
discoloration of the clayish composition for forming the sintered silver
alloy body can be reliably prevented. Here, examples of the metallic
copper included in the powder constituent can include metallic copper
powder, and metallic copper included in the alloy powder of Ag and Cu.
[0024] (6) Furthermore, in the clayish composition for forming the
sintered silver alloy body according to any one of (1) to (5), the copper
oxide powder further includes copper (I) oxide (Cu2O), the total
amount of copper (II) oxide and copper (I) oxide in the powder
constituent is preferably 54 mass % or less with respect to the entire
powder constituent.

[0025] If the powder constituent includes a large amount of oxides, such
as CuO or Cu2O, removal of the binder and reduction by CO become
difficult, therefore there is a concern of adversely affecting the
sintering property when firing the clayish composition for forming the
sintered silver alloy body. In addition, Cu2O is also gradually
changed to CuO, but discoloration is not as abrupt as when the metallic
copper is added. From the above facts, in a case in which the powder
constituent includes copper (I) oxide, the total amount of copper (II)
oxide and copper (I) oxide in the powder constituent is preferably 54
mass % or less with respect to the entire powder constituent. [0026] (7)
In addition, in the clayish composition for forming the sintered silver
alloy body according to any one of (1) to (6), the average particle
diameter of the copper oxide powder is preferably 1 μm or more and 25
μm or less.

[0027] In this case, the mechanical strength, elongation or the like of
the sintered silver alloy body obtained by firing the clayish composition
for forming a sintered silver alloy body can be improved. [0028] (8)
Furthermore, at least one of fatty substance and surface active agent,
according to necessity, may be added to the clayish composition for
forming the sintered silver alloy body according to any one of (1) to
(7). [0029] (9) In addition, in the clayish composition for forming a the
sintered silver alloy body according to any one of (1) to (8), the binder
may include at least one kind or two or more kinds of binders selected
from the group consisting of a cellulose-based binder, a polyvinyl
compound-based binder, an acrylic compound-based binder, a wax-based
binder, a resin-based binder, starch, gelatin and flour. In addition,
among the above, the binder most preferably includes a cellulose-binder,
particularly, a water-soluble cellulose.

[0030] The kind of the surface active agent is not particularly limited,
and a general surface active agent may be used.

[0031] Examples of the fatty substance can include an organic acid (oleic
acid, stearic acid, phthalic acid, palmitic acid, sebacic acid, acetyl
citrate, hydroxybenzoic acid, lauric acid, myristic acid, caproic acid,
enanthic acid, butyric acid and capric acid), organic acid ester (organic
acid ester including a methyl group, an ethyl group, a propyl group, a
butyl group, an octyl group, a hexyl group, a dimethyl group, a diethyl
group, an isopropyl group or an isobutyl group), higher alcohols
(octanol, nonanol, decanol), polyhydric alcohols (glycerin, arabinitol,
sorbitan), or ether (dioctyl ether, didecyl ether). [0032] (10) The
present powder used for the clayish composition for forming the sintered
silver alloy body according to any one of (1) to (9) is characterized by
including the silver powder and the copper oxide powder. [0033] (11) In
addition, the powder for the clayish composition for forming the sintered
silver alloy body according to (10) preferably includes copper (II) oxide
powder (CuO powder) as the copper oxide powder. [0034] (12) Furthermore,
the powder for the clayish composition for forming the sintered silver
alloy body according to (10) or (11) preferably includes the CuO powder
as the copper oxide powder in a range of from 4 mass % to 35 mass % with
respect to the entire powder for the clayish composition, and the amount
of Ag element is preferably from 46 mass % to 97 mass % with respect to
the total metal component, which does not include the oxygen in the
powder for the clayish composition. [0035] (13) In addition, the powder
for the clayish composition for forming the silver alloy body according
to any one of (10) to (12) preferably includes CuO powder as the copper
oxide powder in a range of from 12 mass % to 35 mass % with respect to
the entire powder for the clayish composition, and the amount of Ag
element is preferably from 46 mass % to 90 mass % with respect to the
total metal component, which does not include the oxygen in the powder
for the clayish composition. [0036] (14) Furthermore, the powder for the
clayish composition for forming the sintered silver alloy body according
to any one of (10) to (13) preferably includes metallic copper, and an
amount of the metallic copper in the powder for the clayish composition
is preferably 2 mass % or less with respect to the entire powder for the
clayish composition. [0037] (15) In addition, the powder for the clayish
composition for forming the sintered silver alloy body according to any
one of (10) to (14) preferably further includes copper (I) oxide, and the
total amount of copper (II) oxide and copper (I) oxide in the powder for
the clayish composition is preferably 54 mass % or less with respect to
the entire powder for the clayish composition. [0038] (16) Furthermore,
in the powder for the clayish composition for forming the sintered silver
alloy body according to any one of (10) to (15), the average particle
diameter of the copper oxide powder is preferably 1 μm or more and 25
μm or less.

[0039] According to the powder for the clayish composition for forming the
sintered silver alloy body with the above constitution, the
above-described clayish composition for forming the sintered silver alloy
body can be constituted, therefore the discoloration of the clayish
composition for forming the sintered silver alloy body can be reliably
prevented. [0040] (17) The method for manufacturing the clayish
composition for forming a sintered silver alloy body according to the
present invention is characterized by mixing the powder for the clayish
composition for forming the sintered silver alloy body according to any
one of (10) to (16), and binding agent including a binder and water.

[0041] According to the method for manufacturing the clayish composition
for forming a sintered silver alloy body with such a constitution, it is
possible to manufacture a clayish composition for forming a sintered
silver alloy body which includes the copper oxide powder and is difficult
to be discolored. [0042] (18) The sintered silver alloy body according to
the present invention is characterized by being obtained by firing the
clayish composition for forming a sintered body according to any one of
(1) to (9).

[0043] According to the sintered silver alloy body with such a
constitution, since the sintered silver alloy body is a body obtained by
firing a clayish composition for forming a sintered silver alloy body
with the above-described constitution, compared with a body obtained by
firing silver clay made of pure Ag powder, the mechanical strength can be
improved. That is, a sintered silver alloy body obtained by heating and
firing the above clayish composition for forming a sintered silver alloy
body has excellent mechanical strength, elongation, and the like. [0044]
(19) The method for manufacturing the sintered silver alloy body
according to the present invention is characterized by obtaining a
sintered silver alloy body by making the clayish composition for forming
a sintered silver alloy body according to any one of (1) to (9) into an
arbitrary shape so as to produce an object, and by firing in a reduction
atmosphere or a non-oxidizing atmosphere after drying the object.

[0045] According to the method for manufacturing the sintered silver alloy
body with the above constitution, it is possible to manufacture a
sintered silver alloy body with excellent mechanical strength,
elongation, and the like by making the above clayish composition for
forming a sintered silver alloy body and then conducting a drying
treatment and a heating and firing treatment.

[0046] Here, as described in the above, in a case in which the clayish
composition for forming a sintered silver alloy body includes CuO powder
at an amount of 12 mass % or more with respect to the entire powder
constituent, the binder included in the clayish composition for forming a
sintered silver alloy body can be combusted and thus removed by using the
oxygen in CuO, therefore a pre-baking process for removing the binder can
be omitted. [0047] (20) The method for manufacturing the sintered silver
alloy body according to (19) preferably includes manufacturing a sintered
silver alloy body by firing the object in a reduction atmosphere or a
non-oxidizing atmosphere at a firing temperature of from 650° C.
to 830° C. for a time of from 15 minutes to 120 minutes after
drying the object.

[0048] According to the method for manufacturing the sintered silver alloy
body with such a constitution, it is possible to reliably conduct
sintering to burn off and thus remove the binder by limiting the firing
conditions of the object of the clayish composition for forming a
sintered silver alloy body to the above. [0049] (21) Furthermore, in the
method for manufacturing the sintered silver alloy body according to (19)
or (20), the object has portions with a thickness of 5 mm or more,
therefore the rate of rising temperature from room temperature to the
above firing temperature is preferably in a range of from 15°
C./min to 80° C./min when firing the object in a reduction
atmosphere or a non-oxidizing atmosphere after drying the object.

[0050] In general, for a relatively thick object of the clayish
composition for forming a sintered silver alloy body with a thickness of
5 mm or more, it is extremely difficult to combust and remove the binder
inside the object, therefore it is necessary to decrease the rate of
rising temperature to the firing temperature. This is because oxygen to
combust the binder is supplied from the surface layer of the object;
therefore the binder is not sufficiently combusted inside the object.

[0051] Here, a thickness of 5 mm or more means that the diameter of at
least one inscribed sphere present inside the object is 5 mm or more.

[0052] Here, since the method for manufacturing the sintered silver alloy
body according to the present invention uses the clayish composition for
forming a sintered silver alloy body including copper oxide powder as
described above, the binder inside the object can be reliably combusted
by using oxygen in the copper oxide powder. Therefore, even when a
relatively thick object of the clayish composition for forming a sintered
silver alloy body with a thickness of 5 mm or more is fired at a
relatively fast rate of temperature rise from room temperature to the
firing temperature set in a range of from 15° C./min to 80°
C./min, it is possible to manufacture a sintered silver alloy body that
is sintered far enough into the inside.

[0054] Particularly, in the case of including copper (II) oxide (CuO) as
the copper oxide powder, since the content of oxygen is relatively high,
sintering can be accelerated, and a relatively thick object of the
clayish composition for forming a sintered silver alloy body with a
thickness of 5 mm or more can be reliably sintered. [0055] (22) In
addition, the method for manufacturing the sintered silver alloy body
according to any one of (19) to (21) preferably includes firing in a
state in which the object is buried in activated carbon.

[0056] According to the method for manufacturing the sintered silver alloy
body with such a constitution, the sintering of the object can be
accelerated by the reduction of the activated carbon.

Effects of the Invention

[0057] According to the clayish composition for forming a sintered silver
alloy body according to the present invention, with the above
constitution and effects, it is possible to suppress the discoloration of
the clayish composition for forming a sintered silver alloy body and to
improve the mechanical strength, elongation, and the like of a sintered
silver alloy body obtained by heating and firing the clayish composition
after making.

[0058] According to the powder for the clayish composition for forming a
sintered silver alloy body according to the present invention, it is
possible to suppress the discoloration of a clayish composition for
forming a sintered silver alloy body by constituting a clayish
composition for forming a sintered silver alloy body using the powder for
the clayish composition for forming a sintered silver alloy body.

[0059] According to the method for manufacturing the clayish composition
for forming a sintered silver alloy body according to the present
invention, it is possible to reliably manufacture the above clayish
composition for forming a sintered silver alloy body.

[0060] According to the sintered silver alloy body according to the
present invention, it is possible to improve the mechanical strength of
the silver sintered body compared with a body obtained by firing silver
clay made of pure Ag powder.

[0061] In addition, according to the method for manufacturing the sintered
silver alloy body according to the present invention, it is possible to
manufacture a sintered silver alloy body with excellent mechanical
strength, elongation, and the like by conducting a drying treatment or
firing under the predetermined conditions after making the object by
using a clayish composition for forming a sintered silver alloy body with
the above constitution.

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] FIG. 1 is a view schematically showing a method for manufacturing
the clayish composition for forming a sintered silver alloy body
according to an embodiment of the present invention.

[0063]FIG. 2A is a view schematically showing a making process which
makes an object by using the clayish composition in a method for
manufacturing the sintered silver alloy body according to an embodiment
of the present invention.

[0064] FIG. 2B is a view schematically showing a drying process which
dries the object in an electric furnace in a method for manufacturing the
sintered silver alloy body according to an embodiment of the present
invention.

[0065] FIG. 2C is a view schematically showing a firing process which
fires the object in the electric furnace in a method for manufacturing
the sintered silver alloy body according to an embodiment of the present
invention.

[0066] FIG. 2D is a view schematically showing a conducting post
processing on the silver sintered body obtained by the firing in a method
for manufacturing the sintered silver alloy body according to an
embodiment of the present invention.

[0067]FIG. 3 is a view showing the results of an X-ray diffraction
analysis on the copper-containing oxide powder obtained by oxidizing
metallic copper powder.

BEST MODE FOR CARRYING OUT THE INVENTION

[0068] Hereinafter, an embodiment of the clayish composition for forming a
sintered silver alloy body, a powder for the clayish composition for
forming a sintered silver alloy body, a method for manufacturing the
clayish composition for forming a sintered silver alloy body, a sintered
silver alloy body and a method for manufacturing the sintered silver
alloy body according to the present invention will be described with
appropriate reference to the accompanying drawings.

[0069] Meanwhile, in the present embodiment, the clayish composition for
forming a sintered silver alloy body and the powder for the clayish
composition for forming a sintered silver alloy body will be described
with names of `silver clay` and `powder for silver clay`, respectively.
Furthermore, a sintered silver alloy body will be described with names of
"sintered body" or "silver sintered body".

[0070] (Powder for Silver Clay)

[0071] The powder for silver clay according to the present embodiment
includes a silver-containing metal powder including silver (silver
powder) and a copper-containing oxide powder including copper (copper
oxide powder).

[0072] By using such a powder for silver clay, adding the below-described
additives, and kneading the mixture so as to constitute silver clay, for
a silver sintered body obtained by heating and firing, it is possible to
obtain effects that improve the mechanical strength, elongation, and the
like of the silver sintered body and to suppress the discoloration of the
silver clay.

[0073] The powder for silver clay according to the present embodiment
preferably uses CuO powder as the copper-containing oxide powder. In
addition, Ag powder, Ag--Cu alloy powder or the like may be applied as
the silver-containing metal powder.

[0074] Additionally, it is preferable to include CuO powder in a range of
from 4 mass % to 35 mass % with respect to the entire powder constituent
for silver clay, and the amount of Ag element is preferably from 46 mass
% to 97 mass % with respect to the entire metal elements in the powder
constituent.

[0075] Furthermore, it is preferable to include CuO powder in a range of
from 12 mass % to 35 mass % with respect to the entire powder constituent
for silver clay, and the amount of Ag element is preferably from 46 mass
% to 90 mass % with respect to the entire metal elements in the powder
constituent.

[0076] Here, Cu is an element having an effect of strength improvement by
diffusing into Ag in the silver sintered body during sintering. In a case
in which the amount of CuO powder is from 4 mass % to 35 mass %, the
converted amount of Cu in the silver sintered body is from 3 mass % to 30
mass %. If the amount of Cu in the silver sintered body is less than 3
mass %, there is a concern that it becomes difficult to obtain an effect
of improving the mechanical strength of a silver sintered body obtained
by firing the silver clay. In addition, if the amount of Cu exceeds 30
mass %, there is a concern that the elongation degrades. Therefore, it is
preferable to set the amount of CuO powder in the powder for silver clay
in a range of from 4 mass % to 35 mass % so as to include Cu in the
silver sintered body at a amount of from 3 mass % to 30 mass %.
Meanwhile, the amount of CuO powder is preferably 35 mass % or less in
consideration of the hue of the silver sintered body obtained by firing
the silver clay.

[0077] That is, to make the amount of Cu included in the silver sintered
body in the above range, it is preferable to constitute the silver clay
by adjusting the mixture ratio of the silver-containing metal powder to
the copper-containing oxide powder in consideration of the components of
the silver-containing metal powder including silver and the components of
the copper-containing oxide powder.

[0078] Meanwhile, in the present embodiment, CuO powder was used as the
copper-containing oxide powder, and Ag powder was used as the
silver-containing metal powder. In addition, powder for silver clay was
made to include CuO powder in a range of from 4 mass % to 35 mass % with
respect to the entire powder for silver clay, and have Ag and unavoidable
impurities as the remainder.

[0079] Hereinafter, the particle diameter of Ag powder and CuO powder
included in the powder for silver clay according to the present
embodiment will be described.

[0080] In the present embodiment, the particle diameter of Ag powder and
CuO powder is not particularly limited, but considering a variety of
characteristics, such as formability and the like, in the case of
manufacturing silver clay by adding a binding agent as an additive and
kneading, the particle diameter in the range shown below is preferable.

[0081] The average particle diameter of the Ag powder is preferably 25
μm or less. With the average particle diameter of the Ag powder in the
above range, the hue of a silver sintered body obtained by firing the
silver clay becomes good, and, in addition, the above effect of improving
the mechanical strength, elongation, and the like of a silver sintered
body can be stably obtained.

[0082] If the average particle diameter of the Ag powder exceeds 25 μm,
there are concerns in that the hue of the silver sintered body degrades,
and the effect of improving the mechanical strength decreases. In
addition, if the average particle diameter of the Ag powder exceeds 25
μm, the firing property of the powder degrades, therefore a long
firing time is required, and also there is a possibility of an adverse
effect on the workability of the silver sintered body, which is not
preferable.

[0083] Meanwhile, the lower limit of the average particle diameter is not
particularly limited, but if the average particle diameter of the Ag
powder is 1 μm or less, there is a concern in that the costs become
higher in an industrial sense, and the limitation of an apparatus also
needs to be considered; therefore it is preferable to consider 1 μm as
the lower limit.

[0084] In addition, the average particle diameter of the Ag powder is more
preferably in a range of from 1 μm to 20 μm, and still more
preferably in a range of from 3 μm to 10 μm.

[0085] The average particle diameter of the CuO powder is preferably 25
μm or less. With the average particle diameter of the CuO powder in
the above range, the above effect of improving the mechanical strength,
elongation, and the like of a silver sintered body can be stably
obtained.

[0086] If the average particle diameter of the CuO powder exceeds 25
μm, there is a concern in that it becomes difficult to obtain an
effect of improving the mechanical strength of a silver sintered body. In
addition, if the average particle diameter of the CuO powder exceeds 25
μm, similarly to the above case of the Ag powder, the firing property
of the powder degrades, therefore a long firing time is required, and
also there is a possibility of an adverse effect on the workability of
the silver sintered body, which is not preferable.

[0087] Meanwhile, like the above Ag powder, the lower limit of the average
particle diameter is not particularly established, but from the
viewpoints of the limitation of an apparatus or industrial costs, it is
preferable to consider 1 μm as the lower limit of the average particle
diameter of the CuO powder.

[0088] In addition, the average particle diameter of the CuO powder is
more preferably in a range of from 1 μm to 20 μm, and still more
preferably in a range of from 3 μm to 10 μm.

[0089] Furthermore, in the present embodiment, since the sintering
property is increased when firing an object of the silver clay by
limiting the average particle diameters of the Ag powder and the CuO
powder, which constitute the powder for silver clay, to such a
predetermined particle diameter or less as described above, it is
possible to make the treatment temperature in the below-described firing
a low temperature.

[0090] Meanwhile, as a method to measure the average particle diameter of
the above powder, for example, a well-known microtrack method can be
used. In addition, in the present embodiment, d50 (median diameter) was
considered to be the average particle diameter.

[0091] (Silver Clay)

[0092] Next, the silver clay of the present embodiment will be described.

[0093] The silver clay according to the present embodiment includes the
powder for silver clay with the above constitution, a binder (an organic
binder in the present embodiment) and water.

[0094] For example, the silver clay according to the present embodiment
includes the powder for silver clay with the above constitution in a
range of from 70 mass % to 95 mass %, and, furthermore, a binding agent
including an organic binder and water in a range of from 5 mass % to 30
mass %. Here, other than the organic binder and water, a surface active
agent or fatty substance may be added to the binding agent according to
necessity.

[0095] Since the silver clay include the powder constituent including
chemically stable CuO powder and Ag powder, the discoloration in the
atmosphere is suppressed.

[0096] The organic binder used for the silver clay according to the
present embodiment is not particularly limited, but an organic substance
capable of making a clayish composition by binding the powder for silver
clay can be used. Preferable examples of the organic substance include an
organic substance constituted with at least one kind or two or more kinds
of binders selected from the group consisting of a cellulose-based
binder, a polyvinyl compound-based binder, an acrylic compound-based
binder, a wax-based binder, a resin-based binder, starch, gelatin and
flour. In addition, among the above, the binder most preferably includes
a cellulose-binder, particularly, water-soluble cellulose.

[0097] The surface active agent is not particularly limited, and a general
surface active agent (for example, polyethylene glycol or the like) may
be used.

[0099] Hereinafter, an example of a method for manufacturing the silver
clay according to the present embodiment will be described with reference
to the schematic view shown in FIG. 1.

[0100] The method for manufacturing the silver clay 5 according to the
present embodiment is a method that kneads the powder for silver clay 1
in a range of from 70 mass % to 95 mass %, and a binding agent 2
including the organic binder and water in a range of from 5 mass % to 30
mass %.

[0101] As shown in FIG. 1, in the method for manufacturing the silver clay
5 described in the present embodiment, firstly, each of Ag powder 1A and
CuO powder 1B is fed into an mixing apparatus 50 in a predetermined
amount. At this time, for example, 87.8 mass % of Ag powder 1A (average
particle diameter of 5 μm: a microtrack method; atomized powder) and
12.2 mass % of CuO powder 1B (average particle diameter of 5 μm: a
microtrack method; a reagent manufactured by Kishida Chemical Co., Ltd.
with a purity of 97% or more) are fed.

[0102] Additionally, a powder for silver clay 1 is obtained by mixing each
of the above material powder in the mixing apparatus 50.

[0103] Next, as shown in FIG. 1, a binding agent 2 is added to the powder
for silver clay 1 in the mixing apparatus 50. At this time, the amount of
the binding agent 2 added can be made approximately {total weight of the
powder for silver clay 1 to binding agent 2=9:1}.

[0104] Here, the binding agent 2 includes the organic binder, the fatty
substance and the surface active agent in a ratio of from 11 mass % to 17
mass %: 5 mass % or less: 2 mass % or less with water as the remainder.

[0105] Additionally, silver clay 5 is obtained by mixing and kneading the
powder for silver clay 1 and the binding agent 2 in the mixing apparatus
50.

[0106] (Silver Sintered Body)

[0107] The silver sintered body according to the present embodiment is
obtained by shaping and making an object by using the silver clay 5 with
the above constitution into an arbitrary shape, and then firing it under
the below-described conditions.

[0108] The silver sintered body has excellent mechanical strength,
therefore, for example, even in the case of exerting a large external
force, it is possible to suppress the occurrence of cracking or
rupturing. In addition, since the silver sintered body according to the
present embodiment has an excellent mechanical strength and a high
elongation, for example, even in the case of conducting an additional
process accompanying bending on the silver sintered body after firing, it
is possible to suppress the occurrence of cracking or rupturing.

[0109] Hereinafter, an example of a method for manufacturing the silver
sintered body according to the present embodiment will be described with
reference to the schematic views of FIGS. 2A to 2D.

[0110] The method for manufacturing the silver sintered body 10 according
to the present embodiment is a method that makes an object 51 by using
the silver clay 5 with the above constitution into an arbitrary shape,
then dries the object 51, for example at a temperature of from room
temperature to 150° C. for from 30 minutes to 24 hours, and then
fires the object 51 in a reduction atmosphere or a non-oxidizing
atmosphere at a temperature of from 650° C. to 830° C. for
15 minutes to 120 minutes, thereby manufacturing a silver sintered body
10. Here, as a method that conducts the above firing, for example, a
method that buries the dried object 51 in activated carbon and then
conducts firing at a temperature of from 650° C. to 830° C.
for 15 minutes to 120 minutes can be employed.

[0111] Firstly, as shown in FIG. 2A, the silver clay 5 is shaped and made
into an arbitrary shape by, for example, mechanical working with a
stamper, press molding, extrusion molding, or the like, or manual working
by a worker, thereby making an object 51.

[0112] Next, as shown in FIG. 2B, the object 51 is fed into an electric
furnace 80, and a drying treatment is conducted, thereby removing
moisture or the like.

[0113] The drying temperature at this time is, from the viewpoints of an
effective drying treatment, preferably, for example, in a range of from
room temperature to 150° C. or from about 80° C. to
150° C. In addition, from the same viewpoints, the time of the
drying treatment is, for example, from 30 minutes to 720 minutes, and
more preferably from 30 minutes to 90 minutes, and, as an example, it is
possible to conduct the drying treatment under the conditions of a drying
temperature of about 100° C. and a drying time of about 60
minutes.

[0114] Subsequently, as shown in FIG. 2C, the object 51 is fired so as to
produce a silver sintered body 10. At this time, by using the oxygen in
CuO included in the powder for silver clay, the organic binder included
in the silver clay is combusted, which makes it possible to remove the
organic binder.

[0115] Here, the expression "using the oxygen in CuO" refers to a
phenomenon in which CuO emits oxygen by thermal decomposition during
firing and the oxygen helps the combustion of the organic binder.

[0116] In addition, in the present embodiment, a method is employed that
manufactures a silver sintered body 10 by conducting firing on the object
51 using an apparatus shown in the drawing.

[0117] At this time, firstly, the object 51 is buried in the powdery or
granular activated carbon 61 charged into a ceramic firing container 60.
At this time, it is preferable to ensure a distance from the surface of
the activated carbon 61 in the firing container 60 to the object 51 is10
mm or more in order to fully bury the object 51 and prevent the object 51
from being externally exposed in a case in which the activated carbon is
combust.

[0118] Additionally, the firing container 60, in which the object 51 is
buried in the activated carbon 61, is fed into the electric furnace 80,
and heated at a temperature of from 650° C. to 830° C. as
described above for 15 minutes to 120 minutes so as to conduct firing.

[0119] Because of a reduction atmosphere derived the activated carbon 61,
the firing of the object 51 can be performed, even if the object 51 is
not buried in the activated carbon 61.

[0120] In addition, as shown in FIG. 2D, it is possible to produce a
product by conducting post processing, such as surface polishing,
decorating treatment, or the like, according to necessity, on the silver
sintered body 10 obtained by firing.

[0121] Meanwhile, although the object 51 obtained by using the silver clay
5 and the silver sintered body 10 are shaped into a rough block shape for
the convenience of illustration in the drawings and explanation in the
example shown in FIGS. 2A to 2D, it is needless to say that it is
possible to shape the silver clay 5 and the silver sintered body 10 into
a variety of artistic shapes.

[0122] In addition, the present embodiment describes an example using an
electric furnace in each process of the drying treatment and firing, but
the present invention is not limited thereto, and can employ any
apparatuses, such as a gas heating apparatus or the like, with no
limitation as long as they can maintain the heating conditions.

[0123] As described in the above, according to the powder for silver clay
1, which is the present embodiment, it is possible to improve the
mechanical strength, elongation, or the like of the silver sintered body
10 obtained by conducting a drying treatment after making an object and
then heating and firing by constituting the silver clay 5 using the
powder for silver clay 1 from the above constitution and effect.
Furthermore, since the silver clay 5 includes chemically stable CuO, CuO
is not easily altered in the atmosphere, and the discoloration of the
silver clay 5 can be suppressed.

[0124] In addition, according to the silver clay 5, which is the present
embodiment, since the silver clay 5 is obtained by using and kneading the
powder for silver clay 1 with the above constitution, it is possible to
improve the mechanical strength, elongation, or the like of the silver
sintered body 10 obtained by making an object and then heating and firing
in the same manner as the above. Furthermore, since Cu is included in the
form of CuO, the discoloration of the silver clay 5 can be suppressed.

[0125] Moreover, according to the method for manufacturing the silver
sintered body 10, which is the present embodiment, it is possible to
manufacture a silver sintered body 10 with an excellent mechanical
strength, elongation, or the like by making an object by using the silver
clay 5 with the above constitution, and then conducting a drying
treatment or firing under predetermined conditions.

[0126] Thus far, the embodiment of the present invention has been
described, but the present invention is not limited thereto, and
appropriate modifications can be made as long as they do not depart from
the technical idea of the present invention.

[0127] For example, the embodiment described the powder for silver clay
made of Ag powder and CuO powder, but the powder for silver clay is not
limited thereto, and may be powder for silver clay including Ag--Cu alloy
powder or the like, and copper-containing oxide powder. Alternatively,
the powder for silver clay may include Cu powder or Ag--Cu alloy powder
added in addition to Ag powder and copper-containing oxide powder. In
this case, the metallic copper content included in Cu powder and Ag--Cu
allow powder is preferably 2 mass % or less with respect to the entire
powder constituent for silver clay. Thereby, the discoloration of the
silver clay can be reliably suppressed. The metallic copper content in
the powder for silver clay may be in a range of from 0.01 mass % to 2
mass %.

[0128] In addition, other than Ag powder and CuO powder, Cu2O powder
may be used. In this case, the total amount of copper (II) oxide (CuO)
and copper (I) oxide (Cu2O) in the powder for silver clay is
preferably 54 mass % or less with respect to the entire powder for silver
clay. Thereby, it is possible to reliably accelerate sintering by using
oxygen in a copper-containing oxide. The total amount of copper (II)
oxide and copper (I) oxide in the powder for silver clay may be in a
range of from 0.01 mass % to 54 mass %.

EXAMPLES

Example 1

[0129] Hereinafter, the clayish composition for forming a sintered body,
powder for the clayish composition for forming a sintered body, method
for manufacturing the clayish composition for forming a sintered body,
silver sintered body and method for manufacturing the silver sintered
body according to the present invention will be described in more detail
by showing examples, but the present invention is not limited to the
examples.

Examples of the Present Invention

[0130] Firstly, powder for the clayish composition for forming a sintered
body (hereinafter, referred to as `powder for silver clay`) was
manufactured in the following order. In the manufacturing of the powder
for silver clay, Ag powder (average particle diameter of 5 μm: a
microtrack method; atomized powder) and CuO powder (average particle
diameter of 5 a microtrack method; a reagent manufactured by Kishida
Chemical Co., Ltd. with a purity of 97% or more) were mixed using a
mixing apparatus as shown in FIG. 1 so as to obtain powder for silver
clay including the remainder of Ag and CuO of 4 mass % (Example 1 of the
present invention), the remainder of Ag and CuO of 9.2 mass % (Examples 2
and 9 of the present invention), the remainder of Ag and CuO of 12.2 mass
% (Examples 3, 7 and 8 of the present invention), the remainder of Ag and
CuO of 35 mass % (Example 4 of the present invention), the remainder of
Ag and CuO of 3 mass % (Example 5 of the present invention) and the
remainder of Ag and CuO of 40 mass % (Example 6 of the present
invention).

[0131] In addition, as Examples 17 and 18 of the present invention, powder
for silver clay was obtained by mixing copper-containing oxide powder
manufactured by heating and oxidizing metallic copper powder (average
particle diameter of 20 μm: a microtrack method; reduced powder
manufactured by Fukuda Metal Foil & Powder Co., Ltd.) in the atmosphere
at 340° C. for 3 hours and Ag powder (average particle diameter of
5 μm: a microtrack method; atomized powder). Meanwhile, the mixture
ratio was 12.2 mass of the copper-containing oxide powder to the
remainder of the Ag powder.

[0132] Here, FIG. 3 shows the results of an X-ray diffraction analysis on
the copper-containing oxide powder manufactured by oxidizing metallic
copper powder using an X-ray diffraction apparatus RINT Ultima (trade
name, manufactured by Rigaku Corporation). The results of the X-ray
diffraction analysis clearly show the peaks of CuO and Cu2O. In
addition, the copper-containing oxide powder manufactured by oxidizing
metallic copper powder appeared black across the entire surface. From
this fact, it was observed that CuO was formed on at least the surface of
the copper-containing oxide powder manufactured by oxidizing metallic
copper powder.

[0133] Next, an organic binder, water, a surface active agent and a fatty
substance were mixed so as to produce a binding agent. Then, the binding
agent was added to the powder for silver clay obtained in the above
order, which was left in the mixing apparatus, and kneaded so as to
manufacture a clayish composition for forming a sintered body
(hereinafter, referred to as `silver clay`).

[0134] Here, for the binding agents in Examples 1 to 7, 9, 17 and 18 of
the present invention, 15 mass % of methyl cellulose, 3 mass % of olive
oil, which is a kind of organic acid, and 1 mass % of polyethylene glycol
were mixed as the organic binder, fatty substance and surface active
agent, respectively, with water as the remainder.

[0135] In addition, 85 mass % of the powder for silver clay and 15 mass %
of the binding agent were kneaded so as to produce the silver clay.

[0136] On the other hand, for the binding agent in Example 8 of the
present invention, 13 mass % of a mixture of water-soluble cellulose
ester (manufactured by Shin-Etsu Chemical Co., Ltd., METOLOSE SM8000) and
potato starch (manufactured by Nippon Starch Chemical Co., Ltd., DELICA
M9) mixed in a ratio of water-soluble cellulose ester to potato starch of
4 to 3 was mixed as the organic binder with the remainder of water.

[0137] In addition, 85 mass % of the powder for silver clay and 15 mass %
of the binding agent were kneaded so as to produce the silver clay.

[0138] Here, an analysis on the amount of Cu included in the obtained
silver clay was carried out. Firstly, the organic binder, surface active
agent, and fatty substance were removed by washing the silver clay in hot
water of 90° C. or more, and then a predetermined amount of
specimen necessary for a quantitative analysis (about 10 g) was taken.
Subsequently, a quantitative analysis of Cu was carried out on the
specimen for analysis by an ICP analysis. As a result, as shown in
Tables. 1 and 2, it was observed that the theoretical amount of Cu mixed
as CuO powder and the actual amount of Cu included in the silver clay
were matched.

[0139] Next, a wire-like object with the dimensions of a diameter of about
1.2 mm and a length of about 50 mm (before firing) and a prismatic object
with the dimensions of a length of about 30 mm, a width of about 3 mm and
a thickness of about 3 mm (before firing) were manufactured by using and
using the silver clay obtained in the above order.

[0140] Subsequently, as shown in FIG. 2B, each object 51 of the wire-like
object and the prismatic object was fed into an electric furnace (ORTON,
manufactured by Evenheat Kiln Inc.) 80 for each example of the present
invention at the same time, and dried under the conditions of a drying
temperature of 100° C. and a drying time of 60 minutes, thereby
removing moisture and the like included in the object 51.

[0141] Meanwhile, FIGS. 2A to 2C show only one prismatic object as the
object 51 and do not show the wire-like object.

[0142] Here, for Examples 1, 2, 5, 7 and 18 of the present invention, a
pre-baking process was carried out in the atmosphere at 500° C.
for 30 minutes using the electric furnace 80 so as to remove the binder.

[0143] Meanwhile, in Examples 3, 4, 6, 8, 9 and 17 of the present
invention, the pre-baking process was not carried out.

[0144] Next, the object 51 for each example of the present invention was
subjected to firing at the same time so as to manufacture a silver
sintered body.

[0145] Specifically, as shown in FIG. 2C, a ceramic firing container 60
having activated carbon 61 charged inside was prepared, and the object 51
was buried in the activated carbon 61. At this time, the distance between
the surface of the activated carbon 61 and the object 51 was about 10 mm.

[0146] In addition, the firing container 60, in which the object 51 was
buried in the activated carbon 61, was put into the electric furnace 80,
and firing was carried out under the conditions of a heating temperature
of 760° C. and a heating time of 30 minutes for all examples of
the present invention, thereby manufacturing the wire-like and prismatic
silver sintered body 10.

COMPARATIVE EXAMPLES

[0147] For Comparative examples 1 and 2, silver clay was manufactured in
the same manner as Examples 1 to 7 of the present invention using an
alloy powder including the remainder of Ag and Cu of 7.5 mass % (average
particle diameter of 33 μm: a microtrack method; atomized powder) as
the powder for silver clay.

[0148] In addition, for Comparative example 3, silver clay was
manufactured in the same manner as Examples 1 to 7 of the present
invention using powder for silver clay in which Ag powder (average
particle diameter of 5 μm: a microtrack method; atomized powder) and
Cu powder (average particle diameter of 20 pin: a microtrack method;
reduced powder manufactured by Fukuda Metal Foil & Powder Co., Ltd.) were
mixed in a ratio of Ag (the remainder) and Cu of 7.5 mass %.

[0149] Furthermore, for Comparative Example 4, silver clay was
manufactured in the same manner as Examples 1 to 7 of the present
invention using silver powder with a diameter of from 1 μm to 15 μm
and a purity of 99.9% as the powder for silver clay.

[0150] Additionally, a wire-like object with the dimensions of a diameter
of about 1.2 mm and a length of about 50 mm (before firing) and a
prismatic object with the dimensions of a length of about 30 mm, a width
of about 3 mm and a thickness of about 3 mm (before firing) were
manufactured by using the obtained silver clay.

[0151] Subsequently, as shown in FIG. 2B, the object 51 of the wire-like
object and the prismatic object was fed into an electric furnace (ORTON,
manufactured by Evenheat Kiln Inc.) 80 for each example of the present
invention at the same time, and dried under the conditions of a drying
temperature of 100° C. and a drying time of 60 minutes, thereby
removing moisture and the like included in the object 51.

[0152] Here, for Comparative Examples 1 and 3, a pre-baking process was
carried out in the atmosphere at 500° C. for 30 minutes using the
electric furnace 80 so as to remove the binder.

[0153] Meanwhile, in Comparative Examples 2 and 4, the pre-baking process
was not carried out.

[0154] Next, the object 51 for each example of the present invention was
subjected to firing at the same time so as to manufacture a silver
sintered body.

[0155] Specifically, as shown in FIG. 2C, the ceramic firing container 60
having activated carbon 61 charged inside was prepared, and the object 51
was buried in the activated carbon 61. At this time, the distance between
the surface of the activated carbon 61 and the object 51 was about 10 mm.

[0156] In addition, the firing container 60, in which the object 51 was
buried in the activated carbon 61, was put into the electric furnace 80,
and firing was carried out under the conditions of a heating temperature
of 800° C. and a heating time of 60 minutes for Comparative
Examples 1 to 3, and the conditions of a heating temperature of
700° C. and a heating time of 10 minutes for Comparative Example
4, thereby manufacturing the wire-like and prismatic silver sintered body
10.

[0157] (Evaluation Method)

[0158] An evaluation test was conducted on the manufactured silver clay
and silver sintered body in the following manner.

[0159] Firstly, regarding the discoloration of the silver clay, a
predetermined amount (10 g) of the silver clay was taken and pinched by
plates covered with a transparent polyethylene film, and then flattened
so as to have a thickness of 3 mm. Additionally, the silver clay was kept
at room temperature in the atmosphere, then whether the silver clay was
discolored or not was visually observed and evaluated.

[0160] As the mechanical properties of the silver sintered body, the
flexural strength, tensile strength, density, surface hardness and
elongation were measured by the following test methods. Meanwhile, the
wire-like sintered body was used for the measurement of tensile strength
and elongation, and the prismatic sintered body was used for the
measurement of flexural strength, density and surface hardness.

[0161] The flexural strength was obtained by measuring a stress trajectory
using an AUTOGRAPH AG-X (manufactured by Shimadzu Corporation) with a
pushing speed of 0.5 mm/min and measuring the peak stress within the
elastic range.

[0162] In addition, the tensile strength was, like the above, obtained by
measuring a stress trajectory using an AUTOGRAPH AG-X (manufactured by
Shimadzu Corporation) with a tension rate of 5 mm/min and measuring the
stress at the moment of rupture of the specimen.

[0164] In addition, the surface hardness was obtained by measuring Vickers
hardness under the conditions of a load of 100 g and a load retention
time of 10 seconds using an AKASHI microhardness tester after polishing
the surface of the specimen.

[0165] Furthermore, the elongation was obtained by measuring a stress
trajectory using an AUTOGRAPH AG-X (manufactured by Shimadzu Corporation)
with a tension rate of 5 mm/min and measuring the elongation at the
moment of rupture of the specimen.

[0166] Tables 1, 2 and 3 show the manufacturing conditions and evaluation
results of Examples 1 to 9, 17 and 18, and Comparative Examples 1 to 4.

[0168] As shown in Tables 1 and 2, it was observed that the silver clay of
Examples 1 to 9, 17 and 18 of the present invention were not discolored
even after being kept at room temperature in an atmosphere for 1 month.

[0169] In addition, it became evident that the silver sintered bodies
obtained by making and firing the object by using the silver clay of
Examples 1 to 8 and 18 of the present invention exhibited higher values
in any of the flexural strength, tensile strength, surface hardness and
density, which are the indices of mechanical strength, and an equal or
higher value even in elongation, compared with those of Comparative
example 4, which used pure Ag.

[0170] Meanwhile, for Example 9 of the present invention, which included
the remainder of Ag and CuO of 9.2 mass %, and were not subjected to a
pre-baking process, firing was insufficient, therefore tensile test and
the like could not been carried out. Likewise, for Example 17 of the
present invention, which used the copper-containing oxide powder obtained
by oxidizing metallic copper, and were not subjected to a pre-baking
process, firing was insufficient, therefore a tensile test and the like
could not been carried out.

[0171] In contrast to the above, it was observed that Examples 3, 4, 6 and
8 of the present invention having a amount of CuO of from 12.2 mass % to
40 mass % could obtain silver sintered bodies with a sufficient strength
even without a pre-baking process for removing the organic binder. It is
assumed that this is because the organic binder is combusted and removed
by the oxygen in the CuO powder in the firing process.

[0172] Here, the carbon concentration and oxygen concentration of the
silver sintered body of Examples 3 and 7 of the present invention was
measured. Here, the carbon concentration was measured by an impulse
furnace heating--infrared ray absorption method. In addition, the oxygen
concentration was measured by a high frequency furnace heating--infrared
ray absorption method. The results are shown in Table 3. It is understood
that the organic binder is combusted and removed even without a
pre-baking process, and that the present invention can be obtained a
sufficient strength of the silver sintered body by comparing Examples 3
and 7 of the present invention in Tables 2 and 3.

[0173] Furthermore, compared with Examples 1 to 4 and 6 to 8 of the
present invention,

[0174] Example 5 of the present invention having a amount of CuO powder of
3 mass % failed to exhibit an effect of a remarkable improvement in the
strength (particularly, flexural strength). In addition, Example 6 of the
present invention having a amount of CuO powder of 40 mass % failed to
show a beautiful silver color when the fired silver sintered body was
polished.

[0175] Furthermore, Example 8 of the present invention using a mixture of
water-soluble cellulose ester and potato starch as the organic binder
also exhibited characteristics and the like similar to those of Examples
3 and 7 of the present invention.

[0176] Meanwhile, it was observed that all the silver clay of Comparative
examples 1 to 3 was discolored after being kept at room temperature in an
atmosphere for 3 days. Here, a tensile test and the like could not be
carried out on Comparative example 2, which had not been subjected to a
pre-baking process, since the organic binder was not sufficiently
removed. It was observed that there was a carbonized phase of the organic
binder inside the silver sintered body of Comparative example 2.

[0177] In addition, it was observed that Comparative example 4 using pure
silver was not discolored, but, compared with Examples 1 to 8 of the
present invention, the flexural strength, tensile strength, surface
hardness and density, which were the indices of mechanical strength, were
liable to be low, therefore being easily deformed.

Example 2

[0178] Next, powder for silver clay was obtained by mixing Ag powder
(average particle diameter of 5gm: a microtrack method; atomized powder)
and CuO powder (average particle diameter of 5 μm: a microtrack
method; a reagent manufactured by Kishida Chemical Co., Ltd. with a
purity of 97% or more) by a mixing apparatus shown in FIG. 1 in a ratio
of Ag (the remainder) and CuO of 12.2 mass %.

[0179] In addition, silver powder with a particle diameter of from 1 μm
to 15 μm and a purity of 99.9% was prepared as the powder for silver
clay.

[0180] Subsequently, a binding agent was added and kneaded to each of the
above powder for silver clay in the same manner as Examples 1 to 7 of the
present invention so as to manufacture silver clay.

[0181] The object 51 of Example 10 and Comparative example 5 of the
present invention were manufactured as cubic objects with a side length
of 10 mm using each of the obtained silver clay. The object 51 from the
silver clay including the powder for silver clay including the remainder
of Ag and CuO of 12.2 mass % is Example 10 of the present invention, and
the object 51 from the silver clay including silver powder with a purity
of 99.9% is Comparative example 5.

[0182] Additionally, the above cubic object 51 was dried at room
temperature for 24 hours and fired so as to manufacture a silver sintered
body 10.

[0183] Specifically, as shown in FIG. 2C, the ceramic firing container 60
having activated carbon 61 charged inside was prepared, and the object 51
was buried in the activated carbon 61. At this time, the distance between
the surface of the activated carbon 61 and the object 51 was about 10 mm.

[0184] In addition, the firing container 60, in which the object 51 was
buried in the activated carbon 61, was put into the electric furnace 80,
and firing was carried out.

[0185] Here, for Example 10 of the present invention, the firing was
carried out with a firing temperature of 760° C., a heating time
of 30 minutes and a rate of temperature rise from room temperature to the
firing temperature of 760° C. in a range of from 15° C./min
to 80° C./min, specifically 30° C./min.

[0186] In addition, for Comparative example 5, the firing was carried out
with a firing temperature of 900° C., a heating time of 120
minutes and a rate of temperature rise from room temperature to the
firing temperature of 900° C. of 30° C./min.

[0187] The density of each of the manufactured silver sintered bodies 10
was evaluated. Evaluation results are shown in Table 4.

[0188] It is observed that the specimen using the silver clay of Example
10 of the present invention has a high density of 9.3 g/cm3 and is
fired far enough into the inside even when the cubic object 51 with a
side length of 10 mm is dried and fired with a rate of temperature rise
from room temperature to the firing temperature (760° C.) of
30° C./min without a pre-baking process.

[0189] On the other hand, the specimen using the silver clay of
Comparative example 5 had a density of about 8.6 g/cm3 despite a
high firing temperature and a long heating time being set, which showed
that firing was insufficient compared with Example 10 of the present
invention.

[0192] Subsequently, a binding agent was added and kneaded to each of the
above powder for silver clay in the same manner as Examples 1 to 7 of the
present invention so as to manufacture silver clay.

[0193] Meanwhile, the amount of CuO and Cu2O in the silver clay can
be measured by conducting an X-ray analysis. Specifically, an X-ray
analysis wad conducted using an X-ray diffraction apparatus RINT Ultima
(manufactured by Rigaku Corporation) after polishing the silver sintered
body obtained by firing the silver clay so as to remove fouling on the
surface.

[0194] As a result of the analysis, it was observed that the mixture ratio
of CuO powder and Cu2O powder in the powder for silver clay of
Examples 11 to 16 of the present invention and the content ratio of CuO
powder and Cu2O powder in the silver clay were identical.

[0195] In addition, for Examples 15 and 16 of the present invention,
prismatic objects with the dimensions of a length of about 30 mm, a width
of about 3 mm and a thickness of about 3 mm (before firing) were
manufactured by using the obtained silver clay. Subsequently, as shown in
FIG. 2B, each object 51 of the prismatic object was fed into an electric
furnace (ORTON, manufactured by Evenheat Kiln Inc.) 80 for each example
of the present invention at the same time, and dried under the conditions
of a drying temperature of 100° C. and a drying time of 60
minutes, thereby removing moisture and the like included in the object
51.

[0196] Here, for Example 16 of the present invention, a pre-baking process
was carried out in the atmosphere at 500° C. for 30 minutes using
the electric furnace 80 so as to remove the binder. In addition, for
Example 15 of the present invention, the pre-baking process was not
carried out.

[0197] Next, the object 51 was subjected to firing so as to manufacture a
silver sintered body.

[0198] Specifically, as shown in FIG. 2C, a ceramic firing container 60
having activated carbon 61 charged inside was prepared, and the object 51
was buried in the activated carbon 61. At this time, the distance between
the surface of the activated carbon 61 and the object 51 was about 10 mm.

[0199] In addition, the firing container 60, in which the object 51 was
buried in the activated carbon 61, was put into the electric furnace 80,
and firing was carried out under the conditions of a heating temperature
of 760° C. and a heating time of 30 minutes, thereby manufacturing
a prismatic silver sintered body 10.

[0200] (Evaluation Method)

[0201] The manufactured silver clay and silver sintered body were
subjected to the following evaluation test.

[0202] For Examples 11 to 16, 19 and 20 of the present invention, the
discoloration of the silver clay was evaluated in the following manner. A
predetermined amount (10 g) of the silver clay was taken and pinched by
plates covered with a transparent polyethylene film, and then crushed so
as to have a thickness of 3 mm. Additionally, the silver clay was kept at
room temperature in the atmosphere, then whether the silver clay was
discolored or not was visually observed and evaluated.

[0206] As shown in Table 5, it was observed that the silver clay of
Examples 11 to 16, 19 and 20 of the present invention were barely
discolored even after being kept at room temperature in an atmosphere for
5 days, and discoloration was suppressed compared with Comparative
examples 1 to 3 shown in Table 1.

[0207] However, it was observed that Examples 12, 14 and 20 of the present
invention having a metallic copper amount of greater than 3 mass % were
discolored after 2 weeks. From this fact, it is preferable to set the
metallic copper content at 2 mass % or less in order to reliably prevent
discoloration of the silver clay.

[0208] In addition, as a result of measuring the density of the silver
sintered bodies of Examples 15 and 16 of the present invention, it is
observed that the density is liable to be lower in Example 16 of the
present invention, which has a total amount of CuO powder and Cu2O
powder of more than 55 mass % and has been pre-baked. On the other hand,
for Example 15 of the present invention having a total amount of CuO
powder and Cu2O powder of 54 mass % or less, the density becomes
relatively high even without being pre-baked.

[0209] From the results of the above-described evaluation tests, it is
evident that the silver clay using the powder for silver clay according
to the present invention can suppress discoloration and obtain a silver
sintered body with excellent mechanical strength, elongation and the
like.